The present invention relates to an improved apparatus and method for recovering solder from dross present on the surface of molten solder where it is exposed to atmospheric air.
In the manufacture of a printed circuit board assembly, there are two commercially important methods of soldering, namely, reflow soldering and wave soldering. The reflow soldering is costly as it employs a solder paste composed of solder particles mixed with flux. In the wave soldering, a wave of solder is caused to well upwardly and contact various portions of a printed circuit board at once. As such, the wave soldering is more economical and provides a higher degree of productivity than the reflow and other processes.
A usual wave soldering machine includes a pair of endless chain conveyors which are driven to advance a printed circuit board at a constant speed from the entrance to exit ends of the machine. While the printed circuit board is held by gripping fingers, the board is first carried to a fluxer where a foam or spray of flux is applied to the underside of the board. The printed circuit board is then carried over preheaters where the temperature of each board is elevated to approximately 110xc2x0 C. to 130xc2x0 C. so as to evaporate excess flux solvent, activate the flux and minimize thermal shock to the printed circuit board. After the printed circuit board is brought to such a preheat temperature, the board passes over a solder reservoir to receive solder. The board is finally transported to a cool down zone where the solder is cooled to solidify. The solder reservoir is equipped with heating elements so as to maintain the molten solder at a preset suitable temperature. A relatively narrow solder nozzle is placed in the solder reservoir to produce a turbulent wave. A relatively wide solder nozzle is provided downstream of the narrow solder nozzle to produce a non-turbulent smooth solder wave. A duct extends within the solder reservoir and has one end connected to each of the nozzles. A pump is disposed in the other end of the duct and connected to a motor through a belt-and-pulley assembly. The pump is activated to transmit pressure energy to the molten solder to cause the solder within the duct to lift and form the solder wave. A flow control panel is mounted within each of the nozzles and has a plurality of apertures. The molten solder is caused to flow in a turbulent fashion in the duct due to the pumping action of the solder pump. The apertures of the flow control panel allows uniform flow of the molten solder above the panel within the nozzle.
A problem with such a conventional wave soldering machine is that a quantity of dross forms on the surface of the molten solder where it is exposed to and contacted with atmospheric air. The formation of such dross brings about a change in the height of the solder wave flowing up through the nozzles. When the dross blocks the apertures of the flow control panel, the height of the solder wave becomes low, with the result that at least some of the electronic components in the printed circuit board are left unsoldered. The wave height suddenly becomes higher when the dross is forced away from the apertures due to the continued pumping action of the solder pump. If the wave height is too high, the solder will flow on the top surface of the printed circuit board and ruin the board. Attachment of the dross itself to the printed circuit board causes a short circuit or deteriorates aesthetic appearance of the finished board.
Dross generation also adds considerably to the cost of the wave soldering process due to the lost value of the solder and the maintenance it imposes. It is necessary from time to time to remove dross from the solder reservoir and repair mechanical parts of the machine as damaged by its abrasive action.
Accordingly, it is an object of the present invention to provide an improved apparatus and method for effectively and economically recovering solder from dross formed in a solder reservoir.
According to one aspect of the present invention, there is provided an apparatus for recovering solder from dross, which comprises a solder reservoir for holding a pool of molten solder. At least one solder nozzle is disposed in the solder reservoir and is adapted to form and direct a solder wave against a substrate to be soldered. The solder nozzle includes opposite side troughs inclined downwardly toward one side of the solder reservoir to direct dross toward a dross zone formed in the first side of the solder reservoir, and a hood is mounted to the solder reservoir and located above the dross zone. An agitator extends across the first side of the solder reservoir and is activated to agitate the dross so as to remove oxides and recover solder from the dross.
The temperature within the dross is much lower than that of the molten solder since the dross present on the surface of the molten solder is exposed to atmospheric air. As such, any solder contained in the dross has a relatively low flowability. To this end, a heating element may be mounted within or outside the hood to heat the interior of the hood to at least 100xc2x0 C. at which the flowability of the solder becomes higher. This promotes removal of the solder from the dross. Preferably, a source of non-oxidizing gas such as nitrogen may be introduced into the interior of the hood through a gas injector so as to provide a substantially oxygen-free atmosphere. Such an atmosphere not only promotes removal of the oxides from the dross, but also inhibits reoxidization of the solder as reclaimed from the dross. Preferably, a heater may be connected between the source of non-oxidizing gas and the gas injector.
According to another aspect of the present invention, there is provided a method for recovering solder from dross present in a solder reservoir within which a pool of molten solder is held. Dross is directed toward a dross zone defined in one side of the solder reservoir and covered by a hood. The dross is agitated to thereby remove oxides and recover solder from the dross. The solder as reclaimed from the dross is returned to the pool of molten solder. The oxides are delivered out of the solder reservoir.
Preferably, the interior of the hood may be heated to a temperature of at least 100xc2x0 C. A non-oxidizing gas may preferably be introduced into the interior of the hood to generate a substantially oxygen-free atmosphere within the hood. The none-oxidizing gas may be selected from nitrogen, carbon dioxide, argon and other inert gases, and hydrogen gas and other reactive gases.
The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.